In the field of image displaying, there is a stereo image display technique. The displayed image varies with the variation of the angle of view and/or the distance of the object in the image so as to provide a sense of view meeting the sensory experience. Usually, a surface of an object in such image is consisting of a plurality of image tiles. When the image is displayed, each of the image tiles is processed with the shading process. However, the more the objects, the more the image tiles, and the more the computation of shading therefore. This is usually the bottleneck of the image processing engine and limiting the frame rate provided by the image processing engine.